This invention involves the continuous preparation of anionic clays and the preparation of Mgxe2x80x94Al solid solutions by heat-treatment of the anionic clay. Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein hydroxyl is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO3xe2x88x92, OH, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, SO42xe2x88x92, SiO32xe2x88x92, CrO42xe2x88x92, BO32xe2x88x92, MnO4xe2x88x92, HGaO32xe2x88x92, HVO42xe2x88x92, ClO4xe2x88x92, BO32xe2x88x92, pillaring anions such as V10O26xe2x88x926 and Mo7O246xe2x88x92, monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulphonates such as laurylsulphonate.
It should be noted that a variety of terms are used to describe the material which is referred to herein as an anionic clay. Hydrotalcite-like and layered double hydroxide are interchangeably used by those skilled in the art. Herein the materials are referred to as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
The preparation of anionic clays has been described in many prior art publications.
Recently, two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized, F. Cavani et al xe2x80x9cHydrotalcite-type anionic clays: Preparation, Properties and Applications,xe2x80x9d Catalysis Todayxe2x80x9d, 11 (1991) Elsevier Science Publishers B. V. Amsterdam.
J P Besse and others xe2x80x9cAnionic clays:trends in pillary chemistry, its synthesis and microporous solidsxe2x80x9d(1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
In these reviews the authors state that a characteristic of anionic clays is that mild calcination at 500xc2x0 C. results in the formation of a disordered MgO-like product. The disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. Herein, the disordered MgO-like materials are referred to as Mgxe2x80x94Al solid solutions. Furthermore, these Mgxe2x80x94Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure.
For work on anionic clays, reference is given to the following articles:
Helv. Chim. Acta, 25, 106-137 and 555-569 (1942)
J. Am. Ceram. Soc., 42, no. 3, 121 (1959)
Chemistry Letters (Japan), 843 (1973)
Clays and Clay Minerals, 23, 369 (1975)
Clays and Clay Minerals, 28, 50 (1980)
Clays and Clay Minerals, 34, 507 (1996)
Materials Chemistry and Physics, 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.
European Patent Application 0 536 879 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of a solution of Al(NO3)3 and Mg(NO3)2 to a basic solution containing borate anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.
In U.S. Pat. No. 3,796,792 by Miyata entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, In. etc. In the examples given solutions of the divalent and trivalent cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80xc2x0 C. Example 1 refers to Mg and Al and Example 2 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.
In U.S. Pat. No. 3,879,523 by Miyata entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts followed by washing and drying. It is Important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed and this is provided by NaOH/Na2CO3 solutions. Residual sodium, for example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.
In U.S. Pat. No. 3,879,525 (Miyata) very similar procedures are again described.
In U.S. Pat. No. 4,351,814 to Miyata et al. a method for making fibrous hydrotalcite is described. Such materials differ in structure from the normal plate-like morphology. The synthesis again involves soluble salts. For example, an aqueous solution of a mixture of MgCl2 and CaCl2 is prepared and suitably aged. From this a needle-like product Mg2(OH)3Cl.4H2O precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg2(OH)3Cl.4H2O and the product is again filtered, washed with water, and dried.
In U.S. Pat. No. 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminum nitrate salt solutions. Such solutions being added to a second solution of NAOH and Na2CO3. After precipitation the slurry is filtered and washed twice with distilled water before drying at 125xc2x0 C.
In U.S. Pat. No. 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As an example the solution is made from NAOH, Na2CO3 and Al2O3. In particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105xc2x0 C.
In U.S. Pat. No. 4,904,457 to Misra a method is described for producing hydrotalcites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
The methodology is repeated in U.S. Pat. No. 4,656,156.
In U.S. Pat. No. 5,507,980 to Kelkar et at al a process is described for making novel catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite involves reacting pseudo-boehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxylic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotalcite are presented.
In U.S. Pat. No. 5,439,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotalcite. The method of preparation is again based, on the co-precipitation of soluble salts by mixing with base, for example, by the addition of a solution of RhCl3, Mg(NO3)2 and Al(NO3)3 to a solution of Na2CO3 and NAOH.
Also in U.S. Pat. No. 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminum salts.
In U.S. Pat. No. 5,591,418 to Bhattacharyya a catalyst for removing sulphur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, the anionic clay having been prepared by co-precipitation of a solution of Mg(NO3)2, Al(NO3)3 and Ce(NO3)3. The product again is filtered and repeatedly washed with de-ionized water.
In U.S. Pat. No. 5,114,898/WO 9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulphur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg nitrates or chlorides with a solution of NAOH and Na2CO3. In U.S. Pat. No. 5,079,203/WO 9118670 layered double hydroxides intercalated with polyoxo anions are described, with the parent clay being made by co-precipitation techniques.
In U.S. Pat. No. 5,578,286 in the name of Alcoa a process for the preparation of meixnerite is described. The meixnerite may be contacted with a dicarboxylate or polycarboxylate anion to form a hydrotalcite-like material. In comparative examples 1-3 hydromagnesite is contacted with aluminum trihydrate in a CO2 atmosphere, greater than 30 atmospheres. No hydrotalcite was obtained in these examples.
In U.S. Pat. No. 5,514,316 a method for the preparation of meixnerite is described using magnesium oxide and transition alumina. For comparative purposes aluminum trihydrate was used in combination with magnesium oxide. It was indicated that this method did not work as well as with transition alumina.
U.S. Pat. Nos. 4,454,244 and 4,843,168 describe the use of pillaring anions in anionic clays.
In U.S. Pat. No. 4,946,581 to van Broekhoven co-precipitation of soluble salts such as Mg(NO3)3 and Al(NO3)3 with, and without the incorporation of rare earth salts was used for the preparation of anionic clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.
As indicated in the description of the prior art given-above, there are many applications of anionic clays.
These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular van Broekhoven has described their use in SOx abatement chemistry.
Because of this wide variety of large-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials and which can be carried out in continuous mode are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays. In particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particled materials, the avoidance of alkali metals (which can be particularly disadvantageous for certain catalytic applications): In prior art preparations organic acids were used to peptize alumina. The use of organic acids is expensive and introduces an additional step in the synthesis process and is therefore not cost-effective. Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulphur oxides, etc. are encountered which cause environmental pollution problems. Moreover, none of the preparation methods described in the prior art provide continuous processes for the preparation of anionic clays.
The invention includes a process for producing anionic clays using raw materials which are inexpensive and utilizing such raw materials in a simple process which is extremely suitable to be carried out in continuous mode. The process involves reacting mixtures with or without stirring in water at ambient or elevated temperature at atmospheric or elevated pressure. Such continuous processes can be operated in standard industrial equipment. More specifically, there is no need for washing or filtering, and a wide range of ratios of Mg/Al in the reaction product is possible.
In the continuous process according to the invention an aluminum source and a magnesium source, for instance magnesium oxide or brucite, are fed to a reactor and reacted in aqueous suspension to obtain an anionic clay. This invention involves the use of alumina trihydrate (such as gibbsite, bayerite or nordstrandite) or thermally treated forms thereof as aluminum source. The reaction is carried out at ambient or elevated temperature and ambient or elevated pressure and the reaction mixture results in the direct formation of an anionic clay which can be obtained by simply drying the slurry continuously retrieved from the reactor. The powder X-ray diffraction pattern (PXRD) suggests that the product is comparable to anionic clays made by other standard (batch) methods. The physical and chemical properties of the product are also comparable to those anionic clays made by the other conventional methods. The overall process of this invention is very flexible, enabling a wide variety of anionic clay compositions and anionic clay-like materials involving for example carbonate, hydroxide and other anions to be prepared in an economically and environmental-friendly manner.